Ball Screw And A Method For Manufacturing The Same

ABSTRACT

A ball screw has a nut formed with a helical ball rolling groove on its inner circumference. A screw shaft is formed with a helical ball rolling groove on its inner circumference. The screw shaft is inserted into the nut. A number of balls are rollably contained within a ball rolling passage formed by oppositely arranged helical ball rolling grooves of the nut and the screw shaft, respectively. The nut is made of case hardened steel. The ball rolling groove is formed by cutting and surface hardening, by vacuum carburizing hardening, without any after-processing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/609,191 filed on Oct. 30, 2009, which is a continuation ofInternational Application No. PCT/JP2008/001077, filed Apr. 24, 2008,which claims priority to Japanese Application No. 2007-120661, filed May1, 2007. The disclosures of the above applications are incorporatedherein by reference.

FIELD

The present disclosure relates to ball screws formed with a helical ballrolling groove with a large number of rolling balls used for powersteering apparatus, electric actuators, etc. and a method ofmanufacturing the same.

BACKGROUND

A ball screw is a mechanical element that includes a ball screw shaftand a ball screw nut. The shaft is formed with a helical ball rollinggroove on its outer circumferential surface. The ball screw nut isformed with a helical ball rolling groove on its inner circumferentialsurface. A number of balls are rollably contained within a ball rollingpassage formed by the oppositely arranged helical ball rolling grooves.The ball screw is adapted to convert a rotational motion of the ballscrew shaft or the ball screw nut into an axial translational motion ofthe ball screw nut or the ball screw shaft.

Usually, the ball rolling groove of the ball screw nut is formed bycutting and grinding its inner circumferential surface. Firstly, aprepared bore is formed in a blank by drilling. The helical ball rollinggroove is formed on its circumferential surface of the prepared bore bycutting with the use of a turning tool. After performing a heattreatment, such as carburizing hardening, an outer circumference of theblank is ground by a cylindrical grinding machine. Finally, the surfaceof the ball rolling groove formed by cutting is ground with a grindingstone.

Recently, in ball screw used in automobile actuators, various kinds ofball screws have been proposed to satisfy the demands to reduce theprice of the ball screws. As shown in FIG. 7, one example has beenproposed for a manufacturing method of a ball screw nut. It includes astep of drilling a predetermined inner circumference (bore) at thecenter of a blank (step P1). A step to finish outer and innercircumferences of the blank with the use of a turning tool (step P2). Astep to insert a tapping tool into the inner circumference and cut theball rolling groove thereon (step P3). A step to heat treat (carburizinghardening) the ball rolling groove to form a predetermined hardenedlayer (step P4). A step to shot peen, with steel beads, after the heattreatment (step P5).

Adoption of this prior art manufacturing method eliminates grinding ofthe ball rolling groove using a conventional grinding wheel. Thus, thisreduces the manufacturing cost. In addition, it also eliminates thegeneration of abnormal layers such as a grain boundary oxidation layeron the surface of the ball rolling groove by using the shot peeningprocess. Also, it improves surface characteristics such as surfacehardness and residual compression stress etc. giving influences to thelife of the ball screw. (Japanese Laid-open Patent Publication No.90570/2005.)

However, a problem exists in that regions of the ball rolling groovecannot be directly struck by media, such as steel beads, used in theshot peening process during its performance, as schematically shown inFIG. 8. Thus, it is necessary to eject media again in a reversedirection of ejection. This additional ejecting process not onlyincreases the manufacturing cost but also causes differences in thesurface characteristics between regions struck twice by the media andregions struck only once by the media. Thus, a deterioration in accuracyof the ball rolling groove occurs.

SUMMARY

It is, therefore, an object of the present disclosure to provide a ballscrew and a method for its manufacture that reduces the generation ofgrain boundary oxidation layers and has improved durability.

To achieve the object, a ball screw comprises a nut formed with ahelical ball rolling groove on its inner circumference. A screw shaft isformed with a helical ball rolling groove on its outer circumference.The shaft is inserted into the nut. A number of balls are rollablycontained within a ball rolling passage formed by the oppositelyarranged helical ball rolling grooves of the nut and the screw shaft,respectively. The nut is made of case hardened steel. The ball rollinggroove is formed by cutting and surface hardening by vacuum carburizinghardening, without any after-processing.

The nut is made of case hardened steel. The ball rolling groove isformed by cutting. The ball rolling groove is surface hardened by vacuumcarburizing hardening without any after-processing. Thus, it is possibleto maintain a surface condition of a white-silver color after thehardening treatment similar to that before the hardening treatment andto discharge oxidation components. Thus, no abnormal surface structureand softened layer is generated with suppressing generation of the grainboundary layer as compared with ordinary gas carburizing. Accordingly,it is possible to provide a ball screw with improved resistance againstfatigue and wear and has excellent durability.

The screw shaft is made from case hardened steel. The ball rollinggroove is formed by rolling. It is surface hardened by vacuumcarburizing hardening without any after-processing. This makes itpossible to provide a ball screw with improved resistance againstfatigue and wear and has excellent durability.

A method of manufacturing a ball screw comprises forming a nut with ahelical ball rolling groove on its inner circumference. A screw shaft isformed with a helical ball rolling groove on its outer circumference.The shaft is inserted into the nut. A number of balls are rollablycontained within a ball rolling passage that is formed by oppositelyarranged helical ball rolling grooves of the nut and the screw shaft,respectively. The nut is made of case hardened steel. The ball rollinggroove is formed by the steps of cutting, with use of a universalturning tool, and hardening, by vacuum, carburizing hardening.

Adoption of the method of the present disclosure makes it possible tosuppress the generation of abnormal surface structures, such as a grainboundary oxidation layer and softened layer, as compared with ordinarygas carburizing. Thus, this improves resistance against fatigue andwear. In addition, shot peening performed after heat treatment enablesthe elimination of processes for improving surface roughness such asgrinding. Additionally, it applies residual compression stress and thusobtains stable quality while reducing the total cost.

The whole configuration of the ball rolling groove is formed by using auniversal turning tool with a cutting edge of its nose radius beingsmaller than a radius of curvature of the ball rolling groove. To formthe nut ball rolling groove, the universal turning tool is moved severaltimes by an effective length of the ball rolling groove and bysuccessively shifting, each time, the moving path of the universalturning tool along a circular arc direction of a cross-sectionalconfiguration of the ball rolling groove. This makes it possible toassure a desirable dimension and accuracy, a good surface roughness andeliminates a grinding or cutting process after heat treatment. Thus,this reduces the manufacturing cost.

The ball rolling groove is processed by shot blasting or a tumblertreatment after vacuum carburizing hardening. This makes it possible toeffectively remove burrs generated by the cutting process in the ballrolling groove.

The ball screw comprises a nut formed with a helical ball rolling grooveon its inner circumferential surface. A screw shaft is formed with ahelical ball rolling groove on its outer circumferential surface. Theshaft is inserted into the nut. A number of balls are rollably containedwithin a ball rolling passage formed by oppositely arranged helical ballrolling grooves of the nut and the screw shaft, respectively. The nut ismade of case hardened steel. The ball rolling groove is formed bycutting. Its surface is hardened by vacuum carburizing hardening withoutany after-processing. Thus, it is possible to maintain a surfacecondition of a white-silver color after the hardening treatment similarto before the hardening treatment and to discharge oxidation components.Thus, no abnormal surface structure and softened layer is generated withsuppressing generation of the grain boundary layer as compared withordinary gas carburizing. Accordingly, it is possible to provide a ballscrew with improved resistance against fatigue and wear that hasexcellent durability.

In the method for manufacturing a ball screw, a nut is formed with ahelical ball rolling groove on its inner circumferential surface. Ascrew shaft is formed with a helical ball rolling groove on its outercircumferential surface. The shaft is inserted into the nut. A number ofballs are rollably contained within a ball rolling passage formed byoppositely arranged helical ball rolling grooves of the nut and thescrew shaft, respectively. The nut is made of case hardened steel. Theball rolling groove is formed by the steps of cutting, with the use of auniversal turning tool, and hardening, by vacuum carburizing hardening.Thus, it is possible to suppress the generation of abnormal surfacestructure, such as grain boundary oxidation layer and a softened layeras compared with ordinary gas carburizing. Thus, this improvesresistance against fatigue and wear. In addition, the shot peeningperformed after heat treatment enables the elimination of processes toimprove surface roughness. Additionally, it applies residual compressionstress and thus obtains stable quality and reduces the total cost.

A method for manufacturing a ball screw comprises a nut formed with ahelical ball rolling groove on its inner circumferential surface. Ascrew shaft is formed with a helical ball rolling groove on its outercircumferential surface. The shaft is inserted into the nut. A number ofballs are rollably contained within a ball rolling passage formed byoppositely arranged helical ball rolling grooves of the nut and thescrew shaft, respectively. The nut is made of case hardened steel. Themethod comprises steps of boring, by drilling, a predetermined innercircumference at the center of a blank. The outer and innercircumferences of the blank are finished by a turning tool. The ballrolling groove is point-turned with the use of a universal turning tool.The turned work is hardened by vacuum carburizing hardening. The ballrolling groove is shot blasted after hardening.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a longitudinal sectional view of one embodiment of a ballscrew.

FIG. 2 is a process drawing showing manufacturing steps of the ballscrew.

FIG. 3 is an explanatory view showing a forming process of the ballrolling groove of the nut of the ball screw.

FIG. 4(a)˜(j) are enlarged cross-sectional views each showing a formingstep of the ball rolling groove of the nut.

FIG. 5 is a partially enlarged view showing the ball rolling groove ofFIG. 4.

FIG. 6 is an explanatory view showing a cutting process of the shoulderof the ball rolling groove of FIG. 5.

FIG. 7 is a process drawing showing manufacturing steps of the ballscrew nut of the prior art.

FIG. 8 is a schematic view showing the shot peening treatment of a nutof the prior art.

DETAILED DESCRIPTION

One preferred embodiments of the present disclosure will be described indetail with reference to accompanying drawings.

FIG. 1 is a longitudinal section view of one embodiment of a ball screw.The ball screw includes a nut 1 formed with a helical ball rollinggroove 2 on its inner circumference 1 a. A screw shaft 3 is formed witha helical ball rolling groove 4 on its outer circumference 3 a. Thescrew shaft 3 is inserted into the nut 1. A ball rolling passage isformed by oppositely arranged screw grooves 2, 4. A bridge member 6 isformed with a connecting groove 5 connecting ball rolling grooves 2 ofthe nut 1. A number of balls 7 are rollably contained within the ballrolling passage, formed by oppositely arranged helical ball rollinggrooves 2, 4. The balls 7 can be circulated in an endless manner by thebridge member 6.

The nut 1 and the screw shaft 3 are made of case hardened steel such asSCM430 etc. They are formed with ball rolling grooves 2, 4 by apoint-turning process with use of a universal turning tool 8, describedlater more in detail. The system of ball circulation is not limited tothe bridge member type and thus a return tube type or an end cap typemay be used.

The ball rolling grooves 2, 4 are formed as so-called Gothic arc groovesformed by a combination of two circular arcs each having a slightlylarger radius of curvature than the radius of balls 7. The crosssectional configuration of the ball rolling grooves 2, 4 may be acircular arc configuration. However, the Gothic arc configuration ispreferable since it can set a large contacting angle against the ball 7and a small axial gap. This enables an increase in rigidity against anaxial load and thus suppresses vibration of the ball screw.

Steps of manufacturing the nut 1 of the present disclosure are shown inFIG. 2. First, a predetermined inner circumference 1 a is formed by adrill, etc. at the center of a cylindrical blank (L1). An outercircumference and the inner circumference is of the blank are finishedby a turning tool (L2). A universal turning tool 8 is inserted into theinner circumference 1 a of the nut 1. The universal turning tool 8point-cuts the inner circumference 1 a with NC controlling respectivephases (L3). The blank is heat treated by vacuum carburizing hardening(L4). The ball rolling groove 2 is shot blast treated (L5).

As shown in FIG. 3, the point-cutting is performed by a universalturning tool 8 while holding a cylindrical work piece W(1) by a spindlechuck of a lathe (not shown) and turning it in a predetermineddirection. The universal turning tool 8 is secured to a holder 9 and ismovably supported both in radial and axial directions. The cutting isperformed in accordance with a so-called point-cutting. The universalturning tool 8 has a nose radius R2 of its cutting edge 8 a smaller thana radius of curvature R1 of the ball rolling groove 2. Thus, theformation of the ball rolling groove 2 is performed by moving theuniversal turning tool 8 several times by an effective length of theball rolling groove 2 and by successively shifting, each time, themoving path of the universal turning tool 8 along a circular arcdirection of a cross-sectional configuration of the ball rolling groove2.

The forming process of the ball rolling groove 2 of the nut will bedescribed in detail with reference to FIGS. 4(a)˜(j).

FIG. 4(a) shows a green pipe member before the turning process. Ageneral configuration of the ball rolling groove 2 is formed by thecutting edge 8 a of the universal turning tool 8 by feeding the cuttingedge 8 a successively from FIG. 4(b) to FIG. 4(d) toward an axial centerof the work piece W(1). The dimension of the nose radius R2 of thecutting edge 8 a is set substantially near the radius of curvature R1 ofthe ball rolling groove 2. Thus, it is possible to obtain a generalconfiguration of the ball rolling groove 2 without axially moving thecutting edge 8 a. Thus, this reduces the processing time.

The point-cutting can be started at a stage shown in FIG. 4(e) where theturning process of the ball rolling groove 2 has been proceeded to someextent. As shown in FIGS. 4(e)˜4(j), a whole configuration of the ballrolling groove 2 is formed by moving cutting edge 8 a of the universalturning tool 8 several times by an effective length of the ball rollinggroove 2 and by successively shifting, each time, the moving path of theuniversal turning tool 8 along a circular arc direction of across-sectional configuration of the ball rolling groove 2. In thisembodiment, the cutting edge 8 a of the universal turning tool 8 doesnot interfere with a portion of the ball rolling groove 2 opposing theportion of the ball rolling groove 2 being processed even if the ballrolling groove 2 has a Gothic arc configuration. This is due to the noseradius R2 of the cutting edge 8 a being set smaller than the radius ofcurvature R1 of the ball rolling groove 2.

As shown in FIG. 5, a shoulder 10 of the ball rolling groove 2 is formedwith a circular arc configuration having a predetermined radius ofcurvature “r”. The shoulder 10 is formed by the same universal turningtool 8 as that used for the point-cutting of the ball rolling groove 2.As shown in FIG. 6, the shoulder 10 is formed similarly to thepoint-cutting of the ball rolling groove 2. Thus, the center “O” of thecurvature is positioned on an extension of the nose radius R2 byshifting the moving path several times during movement of the universalturning tool 8 along a central locus “L” of the nose radius R2 of thecutting edge 8 a.

Accordingly, the ball rolling groove 2 can be formed by point-cuttingwith use of the universal turning tool 8. The universal turning tool 8has the nose radius R2 of the cutting edge 8 a smaller than the radiusof curvature R1 of the ball rolling groove 2. The shoulder 10 can becontinuously formed by the same universal turning tool 8 as that usedfor the point-cutting of the ball rolling groove 2. Thus, it is possibleto complete the formation of the ball rolling groove 2 and the shoulder10 by one step point-cutting and to smoothly form a transition betweenthe ball rolling groove 2 and the shoulder 10. This also provides a ballscrew nut 1 with improvements to the smooth circulation of the balls 7,excellent operability and a low manufacturing cost.

In addition, a hardened layer with a hardness of 55˜62 HRC is formed byheat treatment on the surface of the ball rolling groove 2 after it hasbeen formed by point-cutting. The heat treatment is performed by vacuumcarburizing hardening where carburization and quenching are carried outunder less oxygenated conditions. This maintains a surface condition ofa white-silver color after the hardening treatment. This is similar tothe surface condition before the hardening treatment. Additionally, thisdischarges oxidation components. Thus, no abnormal surface structure andsoftened layer is generated with suppressing the generation of the grainboundary layer as compared with ordinary gas carburizing. Accordingly,it is possible to provide a ball screw with improved resistance againstfatigue and wear and has excellent durability. In addition, shot peeningis performed after heat treatment. This eliminates processes to improvesurface roughness. Also, shot peening applies residual compressionstress. Thus, this obtains stable quality and reduces the total cost.

Results of a durability test of a ball screw, in accordance with thedisclosure, performed by the applicant are shown in Tables 1 and 2. Aspecimen is a ball screw with a screw shaft diameter of 10 mm, a lead of3 mm of the ball rolling grooves 2, 4, an outer diameter of 2.0 mm ofthe ball 7, a bridge circulation type, and two circulation rows. Theball rolling groove 4 of the screw shaft 3 is formed by rolling andvacuum carburizing hardened without any after process similarly to theball screw nut 1.

The test was performed under conditions of: a double nut constantpressure type using a spring load; an axial load of 690 N; a rotationalspeed of 500 rpm; one-way stroke of 6 mm (reciprocal stroke 12 mm); acircumstance temperature of 100° C. under a blare heating system; and atotal number of reciprocal motion of 200,000 (4×10⁵ rev).

TABLE 1 Nut Appearance Expanded amount of gap Nut hardening No. of nut(μm) Ordinary gas 1 Slight peeling 3.0 carburizing 2 Slight peeling 4.03 Slight peeling 4.0 4 No peeling 2.0 Vacuum 1 No peeling 1.0carburizing 2 No peeling 1.5 3 No peeling 1.0 4 No peeling 0

TABLE 2 Nut Appearance Appearance Nut hardening No. of screw shaft ofball Ordinary gas 1 No peeling No peeling carburizing 2 No peeling Nopeeling 3 No peeling No peeling 4 No peeling No peeling Vacuum 1 Nopeeling No peeling carburizing 2 No peeling No peeling 3 No peeling Nopeeling 4 No peeling No peeling

As apparent from Table 1, it has been found that in vacuum carburizednuts no peeling was generated. Also, the expanded amount of the gap wassuppressed smaller than 1.5 μm or less as compared with the ordinary gascarburized nuts. The resistances against fatigue and wear were alsoimproved. In addition as shown in FIG. 2, it is demonstrated that noabnormality, such as peeling, was not found in the screw shaft 3 andball 7. Also, the vacuum carburized nuts is superior in durability tothe ordinary gas carburized nuts.

The shot blasting treatment is applied to the ball rolling groove 2after heat treatment. The shot blasting treatment is a process thatperforms a surface treatment by striking powdered abrasives onto metal,stone, wood, glass or resin etc. There are two types of shot blastingtreatments. In one type, the abrasives are ejected by compressed air. Inthe other type, the abrasives are mechanically ejected by high speedrotational impeller etc. The air ejecting type is preferable since itenables pin point ejection. In addition there are various kinds andgrain size of abrasives such as an alumina family, a steel family, aglass family etc. The steel family is used in the present disclosure.

The heat treatment is performed by the vacuum carburization. Thus, it ispossible to eliminate the conventional shot peening treatment. Inaddition, it is possible to effectively remove burrs generated by thecutting process around the ball rolling groove 2 or the bridge aperture.This reduces the manufacturing cost of the ball screw and achievessmooth insertion of the bridge member 6 as well as smooth circulation ofballs 7. Burrs may be removed by a tumbler treatment in place of theshot blasting treatment.

The ball screw of the present disclosure can be applied to an automatictransmission of vehicle, an electric-powered brake, electric-poweredpower steering, an engine valve control actuator, and also to anelectric-powered shock absorber and an actuator for controlling a widthbetween CVT pullies.

The present disclosure has been described with reference to thepreferred embodiment. Obviously, modifications and alternations willoccur to those of ordinary skill in the art upon reading andunderstanding the preceding detailed description. It is intended thatthe present disclosure be construed to include all such alternations andmodifications insofar as they come within the scope of the appendedclaims or their equivalents.

What is claimed is:
 1. A method for manufacturing a ball screwcomprising: a nut formed with a helical ball rolling groove on its innercircumference, the nut is made of case hardened steel; a screw shaftwith a helical ball rolling groove formed on its outer circumference,the screw shaft inserted into the nut; a number of balls rollablycontained within a ball rolling passage formed by oppositely arrangedhelical ball rolling grooves of the nut and the screw shaft,respectively, comprising the steps of: cutting the nut ball rollinggroove with a universal turning tool; hardening the nut ball rollinggroove by vacuum carburizing hardening.
 2. The method for manufacturingthe ball screw of claim 1, further comprising forming a wholeconfiguration of the ball rolling groove using a universal turning toolwith a cutting edge having its nose radius being smaller than a radiusof curvature of the ball rolling groove by moving the universal turningtool several times by an effective length of the ball rolling groove,and by successively shifting, each time, the moving path of theuniversal turning tool along a circular arc direction of across-sectional configuration of the ball rolling groove.
 3. The methodfor manufacturing a ball screw of claim 1, further comprising shotblasting the nut ball rolling groove after the vacuum carburizinghardening.
 4. The method for manufacturing a ball screw of claim 2,further comprising shot blasting the nut ball rolling groove after thevacuum carburizing hardening.
 5. The method for manufacturing a ballscrew of claim 1, further comprising exposing the nut ball rollinggroove to a tumbler treatment after the vacuum carburizing hardening. 6.The method for manufacturing the ball screw of claim 2, furthercomprising forming a whole configuration of the ball rolling grooveusing a universal turning tool with a cutting edge having its noseradius being smaller than a radius of curvature of the ball rollinggroove by moving the universal turning tool several times by aneffective length of the ball rolling groove, and by successivelyshifting, each time, the moving path of the universal turning tool alonga circular arc direction of a cross-sectional configuration of the ballrolling groove.